1. Field of the Invention
This invention relates to novel volatile liquid reagents which can replace less satisfactory solid sources in film deposition processes such as chemical vapor deposition (CVD), spray coating, spin coating or sol-gel deposition. These liquid reagents can be used for deposition of metals or metal-containing materials, such as metal oxides.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a widely-used process for forming solid materials, such as coatings or powders, from reactants in the vapor phase. Comprehensive reviews of CVD processes have been given recently in "CVD of Nonmetals," W. S. Rees, Jr., Editor, VCH Publishers, Weinheim, Germany, 1996; "CVD of Compound Semiconductors," A. C. Jones and P. O'Brien, VCH, 1996; and "The Chemistry of Metal CVD," T. Kodas and M. Hampden-Smith, Editors, VCH, 1994.
In CVD processes, a reactant vapor may be created by heating a liquid to a sufficiently high temperature and bubbling a flow of a carrier gas through the liquid, to transport the vapor into the CVD chamber. In a low-pressure CVD system, the carrier gas may be omitted, and the vapor may flow directly from the bubbler into the low-pressure CVD chamber.
In order for a CVD process to function successfully, it is necessary to create a vapor containing controlled amounts of suitably reactive chemicals. Solids can be used as sources of vapor in CVD processes. However, when solids are used in a bubbler, the rate of vapor production by sublimation of a solid is not easily reproducible, because the amount of vapor produced often depends on the particle size and shape, which change as the sublimation process continues. Thus the vapor concentration can change in an uncontrolled way, thereby changing the growth rate and/or the composition of materials made by the CVD process. Also, different batches of solid may have different sizes and shapes of particles, so that the results of a CVD process may change when a new batch of solid precursor is placed in the system. These difficulties are particularly evident in the currently-used CVD precursors for barium, strontium and calcium, which are reviewed by W. A. Wojtczak, P. F. Fleig and M. J. Hampden-Smith, in Advances in Organometallic Chemistry, vol. 40, pp. 215-340 (1996).
Another problem with solids is that their rate of sublimation can be altered by small amounts of contamination on their surfaces. In contrast, liquid surfaces tend to be refreshed by motion of the liquid, so that they tend to evaporate at a reproducible rate even in the presence of small amounts of contaminants.
Some solid materials show different vapor pressures, depending on the history of how the particular sample was prepared. For example, aluminum isopropoxide has been used to deposit aluminum oxide films, for example by J. A. Aboaf in the Journal of the Electrochemical Society, vol. 114, pp. 948-952 (1967). Solid aluminum isopropoxide exists in a number of isomeric forms, ranging from dimers to trimers to tetramers to polymers of various lengths. The rates of interconversion between isomeric forms are slow, often taking days. The vapor pressures of these isomers vary widely. Thus it is very difficult to regulate or predict the vapor pressure of any particular sample of aluminum isopropoxide, and the deposition rate of aluminum oxide from this solid source is not reproducible. In comparison, liquids usually exist in only one reproducible form at any given temperature and pressure.
Another difficulty with solids is that rates of sublimation are often low, so that sufficiently high vapor concentrations cannot be produced. For example, aluminum 2-ethylhexanoate has been proposed as a source for CVD of aluminum oxide, by T. Maruyama and T. Nakai in Applied Physics Letters, vol.58, pp.2079-2080 (1991). This solid source material has a very low vapor pressure, which limits the deposition rate to very low values. In comparison, liquids often have higher vapor pressures than solids.
Another practical difficulty with solids is that transferring them between containers is less convenient than pumping liquids.
Thermal decomposition of solids is another problem that often affects the reproducibility of solid vapor sources. For example, the solid beta-diketonates of barium and strontium gradually decompose at their vaporization temperatures, so that the amount of vapor generated decreases with time. Thermal decomposition is also a potential problem for liquid sources, but its effect may be minimized for liquids by rapid or "flash" vaporization. This can be accomplished by pumping the liquid at a steady, controlled rate into a hot region in which the liquid vaporizes quickly. In such a "direct liquid injection" (DLI) system, each part of the liquid is heated for only a short time, and its vapor can be formed without significant decomposition even from thermally sensitive liquids. Another advantage of a DLI system is that multicomponent mixtures can be vaporized in a fixed and reproducible ratio, even if the components differ in volatility. Because of these advantages, DLI systems are becoming more widely used in CVD processes.
Solid sources can be used in DLI vapor sources if a suitable liquid solvent can be found to dissolve the solid. However, solvents can introduce other difficulties, such as increased flammability, toxicity or corrosiveness, and an increased volume of gaseous byproducts must be removed from the exhaust gases to avoid pollution. These difficulties with a solvent can be minimized if the solid is highly soluble in the solvent, so that only a small amount of solvent is needed to form the liquid solution.
Because of all these difficulties, solid sources of vapor are seldom used in commercial CVD processes. Either liquids, or solids that are highly soluble in a liquid solvent, are more convenient, and more commonly used in the practice of CVD. Creating this vapor from a liquid source would be much more reproducible and convenient than creating it from a solid source; however, there are few practical compounds which may be used for this purpose.
Beta-diketonates are one class of solid compounds that can be formed from almost every metal. Beta-diketonates are known for zinc, cadmium, mercury, aluminum, gallium, indium, thallium, tin, lead, antimony, bismuth, lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, technetium, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, cerium, the other rare earth metals and the actinide metals. A thorough review of beta-diketonate compounds was written by A. R. Siedle, in "Comprehensive Coordination Chemistry" (Pergamon Press, Oxford, 1987, vol. 2 chapter 15.4). Almost all of the known beta-diketonate compounds are solids at room temperature, and thus are not very convenient for use as vapor sources for CVD processes.
The vapor pressures of some of the solid beta-diketonates are low, even at temperatures high enough to cause their thermal decomposition. This problem is acute for the beta-diketonates of alkaline earth metals. Their vapor pressures may be increased by vaporizing them in the presence of amines (R. G. Gordon et al., U.S. Pat. No. 5,139,999, 1992), ethers (Miller et al., U.S. Pat. No. 4,501,602, 1985; Timmer et al., U.S. Pat. No. 5,248,787, 1993; Kirlin et al., U.S. Pat. No. 5,280,012, 1994) orthioethers (Kirlin et al., U.S. Pat. No. 5,225,561, 1993). However, even when bound to these Lewis bases, the beta-diketonates disclosed in these references are still solids at room temperature.